I have been on the same hunt.
It looks as thought from what I have found is all dry camping. All state OHV's (Hollister, Carnagie, ETC.) for the most part are NO hook ups at all, any FED forest land... same thing. Frank Reigns park down by Tracy, CA has some full hook up spots and thats about the only one that I know of and its a medium sized ohv park. The best ATV riding that I have found would be around Sac is Eldarado forest near Ice House road on HWY 50 all dry camping and some spots are rough to get in with a 40ft trailer - steeeep inclines, tight roads, one way roads, ETC.
I would like to see other peoples input of good riding/camping spots!!
We have ToyHaulers ...now lets put them to use!!! lol